Xanthogranulomatous Pyelonephritis in an Allograft Kidney Biopsy: A Rare Occurrence and a Potential Pitfall

Introduction: The differential diagnosis of the allograft renal biopsy is fairly routine, but occasionally a confounding diagnosis is present but not made. Case Description: A 45-year-old man with end-stage-kidney disease of unknown etiology underwent second deceased donor kidney transplant .Flow cytometric cross matches from immediate pre-transplantation serum were T and B cell positive. Given very high HLA risk and DSA HLA-B27 (MFI~3000) he was treated with 3 sessions of plasmapheresis. Post-transplantation serum were T and B-cell negative with repeat DSA after plasmapheresis showing weak DSA to HLA-B27. Creatinine trended down to 1-1.3 mg/dL with minimal proteinuria, but 2.5 months later developed fever to 101F and abdominal pain with creatinine increase to 2.2 mg/dL. Kidney transplant sonogram was unremarkable. Initial urinalysis showed mild proteinuria and leukocyturia.Urine culture grew enterococcus faecalis and treatment with ampicillin was begun (treatment had been initiated with steroids, because of high risk). The biopsy showed findings consistent with Grade 1B acute cellular rejection and patient continued steroid therapy. Renal function normalized and has remained excellent with Cr 1.1-1.2 mg/dL. EM and later immunoperodixase (CD68,163) studies disclosed that the infiltrating cell population in the biopsy was almost completely histiocytes. Discussion: Review of the literature showed that histiocyte participation in cellular rejection is usually limited, but does correlate with rejection severity. This degree of histiocyte infiltration would not be generally seen in cellular rejection.Tissue Gram stain and EM revealed coccal forms. PCR detected and identified enterococcus faecalis in the tissue sample. Although xanthogranulomatous pyelonephritis has been identified in the transplant kidney, this finding has been rarely described in the allograft biopsy and presents a potential pitfall in the diagnosis of rejection.